Septoria leaf spot is a fungal disease of the foliage. It shows up as small, roughly round lesions, pale in the middle and outlined by a darker rim, and it almost always begins on the oldest leaves low in the plant before it works its way up. As the lesions become more numerous they run together, the leaf yellows and falls off, and the plant gradually loses the leaf surface it uses to feed itself. On cannabis the disease is usually caught late because the lower inner canopy is the part growers look at least.
What causes it on cannabis
On hemp and drug-type cannabis the pathogen reported by extension plant pathologists is Septoria cannabis. Septoria species tend to be picky about their hosts. The University of Kentucky fact sheet gives the tomato and wheat pathogens as examples and states that Septoria cannabis is known to cause disease only in hemp. That matters for a mixed garden: the Septoria spots on your tomatoes are a different fungus (Septoria lycopersici) and are not a known source of infection for cannabis, although the same wet conditions favour both.
The Kentucky sheet describes Septoria as the most commonly reported leaf spot of field-grown hemp in that state, and Rutgers lists it as one of the most frequently observed leaf spots in New Jersey hemp trials.
Symptoms, step by step
- Small, irregular brown spots with a bright yellow margin appear first on lower leaves and inside the canopy.
- Spots grow to roughly a quarter inch (about 6 mm) and develop grey to white centres.
- Tiny black dots, the spore-producing pycnidia, may be visible inside the spots. Kentucky compares them to pepper flakes.
- Yellow halos spread and merge; whole leaves then yellow quickly and drop.
- In severe early-season cases, plants can be stunted and lose vigour.
How to tell it from other leaf spots and deficiencies
Several things put spots on cannabis leaves, and Utah State lists nutrient deficiencies and insect feeding as the main look-alikes. A 10x hand lens and a look at where on the plant the problem started will settle most cases.
| What you see | Most likely | Key difference |
|---|---|---|
| Brown spots with bright yellow halos, grey-white centres, black specks inside, starting low and inside the canopy | Septoria leaf spot | Pycnidia (black specks) inside the spot; yellow halo expands outward |
| Tiny dark spots with tan centres and dark brown margins | Bipolaris (hemp leaf spot) | Rutgers describes darker, necrotic spots rather than a bright yellow halo |
| Circular to elliptical grey spots with dark brown margins | Cercospora leaf spot | Utah State describes grey lesions with dark margins |
| Brown spots with concentric rings, like a target (on tomato) | Early blight, Alternaria solani | Ringed “target” pattern; Septoria spots have no rings |
| Yellowing between veins or along margins, symmetrical across leaves, no distinct spots with specks | Nutrient problem | No pycnidia, no expanding halo around a lesion |
| Fine pale stippling, sometimes with insects or webbing on the underside | Insect or mite feeding | Look under the leaf with a lens |
The black specks are the most useful single clue. If you cannot see them with a lens, a diagnostic lab can confirm by looking at spores under a microscope.
Life cycle and why wet weather drives it
Septoria spreads by spores (conidia) produced in large numbers inside the pycnidia. When the leaf is wet, spores ooze out and are moved short distances by splashing water and further by wind-driven rain. From infection to visible spots takes about 7 to 10 days, according to Kentucky, and the cycle runs faster and produces more spores when humidity stays high. As the canopy closes, humidity inside it rises and the disease gets worse, which is why it tends to explode mid-season.
The fungus survives between crops on infected plants and debris. Kentucky reports that under cool, dry conditions pycnidia can stay viable in debris for up to nine months, survival drops sharply in hot weather, and pycnidia buried in soil last less than a month. No alternative host has been identified for Septoria cannabis, but both Kentucky and Rutgers suggest weeds may act as a “green bridge” between seasons.
Rutgers sums up the climate side plainly: in regions with wet, humid summers these foliar diseases will be a persistent problem, and severity depends on how long leaves stay wet from rain or dew and on how susceptible the cultivar is.
How much damage it does
Septoria looks alarming, and in bad years it strips a lot of leaf. Kentucky reports that under extreme conditions plants can lose 50% to 90% of leaves in the lower to mid-canopy. The same sheet notes that no direct biomass or cannabinoid yield losses have been observed, though shipments have sometimes been rejected by processors. Utah State rates the threat level as very low for the same reason. The practical reading is that Septoria is mainly a quality and vigour problem, and a sign that the canopy is staying too wet, rather than a crop killer.
Prevention and IPM
Cultural controls come first. These are the measures both Kentucky and Rutgers put at the top:
- Space plants and rows wider so leaves dry fast after rain or irrigation.
- Thin crowded plants and remove the lowest, shaded growth that stays wet.
- Mulch the soil surface to limit splash from the ground up to the leaves.
- Remove infected debris after harvest, or bury it thoroughly by deep tillage in field settings. Rotate the site where possible.
- Choose cultivars with a record of lower susceptibility; Rutgers points to a Tennessee Extension list of hemp cultivars and their leaf spot response.
- Water at the base and early in the day so foliage does not stay wet overnight.
Scout on a schedule. Utah State suggests starting about a month after planting, concentrating on the older inner foliage, dense plantings, and plants that have been wetted repeatedly.
Remove early lesions. Picking off the first spotted lower leaves and taking them out of the growing area reduces the spore load. Do it when foliage is dry so you do not spread spores on wet hands.
Products. Registration is local, and cannabis labels are restrictive. When Kentucky wrote its fact sheet there were no conventional fungicides labelled for hemp, and Bacillus-based biologicals had shown suppression only in lab and greenhouse tests. Utah State lists a handful of products registered in that state, such as sulfur, hydrogen peroxide, Bacillus strains and potassium soaps, several with restrictions like greenhouse-only use or use before flowering. Treat any list as specific to its jurisdiction. Nothing should go onto flowers that is not explicitly allowed for that stage, and sprays do not repair leaves that are already spotted.
Common mistakes
- Calling it a deficiency and adding more feed. Nutrient problems do not produce lesions with black specks inside. Check with a lens before changing the feed.
- Stripping healthy upper leaves in a panic. The disease starts low. Fix airflow and remove the infected lower leaves; the upper canopy is doing the work.
- Leaving fallen leaves on the ground. Fallen leaves carry pycnidia. Collect them.
- Overhead watering in the evening. Long leaf wetness is exactly what the fungus needs.
- Composting infected debris next to next season’s plot. Survival is long in cool, dry debris; bury it or take it off site.
If the spots do not match any of the patterns above, send a sample to a plant diagnostic lab before spending money on products. Check current local rules on which products may be used on cannabis before applying anything.